Hybrid bit with roller cones and discs

ABSTRACT

A hybrid drill bit includes a bit body having a central bit body axis, a roller cone at an end of the bit body and rotatably coupled to the bit body about a roller cone axis, and a disc at the end of the bit body and rotatably coupled to the bit body about a disc axis. The roller cone axis extends toward the central bit body axis. The disc axis is transverse to a radial of the central bit body axis through a center of the disc, and the disc includes a plurality of disc cutters.

BACKGROUND

The present disclosure relates to hybrid drill bits for drilling a wellbore in a formation, and more particularly to hybrid drill bits with roller cones and discs.

A hybrid drill bit for drilling wellbores generally includes more than one type of cutting structure or cutting element at a drill end of the bit. Cutting structures and cutting elements encounter a variety of formations, objects, and surfaces during drilling, such as hard and soft rock formations, cement plugs and well stops, well tools, and/or other objects and surfaces in a wellbore. Because cutting elements and cutting structures remove material by different mechanisms, e.g., shearing, crushing, cracking, or a combination thereof, some are better suited to cutting particular materials than others.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic partially cross-sectional view of an example well system.

FIG. 2 is a schematic perspective view of an example hybrid drill bit.

FIG. 3A is a schematic end view of an example hybrid drill bit.

FIG. 3B is a schematic side view of an example hybrid drill bit profile.

FIG. 4A is a schematic partially cross-sectional side view of an example disc.

FIG. 4B is a schematic partially cross-sectional side view of an example roller cone.

FIG. 5 is a schematic partial end view of an example hybrid drill bit.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring first to FIG. 1, a well system 10 generally includes a substantially cylindrical wellbore 12 that extends from a wellhead 14 at the surface 16 downward into the Earth into one or more subterranean zones of interest 18 (one shown). The subterranean zone 18 can correspond to a single formation, a portion of a formation, or more than one formation accessed by the well system 10, and a given well system 10 can access one, or more than one, subterranean zone 18. A portion of the wellbore 12 extending from the wellhead 14 to the subterranean zone 18 is lined with lengths of tubing, called casing 20. The depicted well system 10 is a vertical well, with the wellbore 12 extending substantially vertically from the surface 16 to the subterranean zone 18. The concepts herein, however, are applicable to many other different configurations of wells, including horizontal, slanted or otherwise deviated wells, and multilateral wells with legs deviating from an entry well.

A drill string 22 is shown as having been lowered from the surface 16 into the wellbore 12. In some instances, the drill string 22 is a series of jointed lengths of tubing coupled together end-to-end and/or a continuous (i.e., not jointed) coiled tubing. The drill string 22 includes one or more well tools, including a bottom hole assembly 24. The bottom hole assembly 24 can include, for example, a drill bit. In the example shown, the wellbore 12 is being drilled. The wellbore 12 can be drilled in stages, and the casing 20 may be installed between stages.

Referring to FIG. 2, an example hybrid drill bit 100 that can be used in the bottom hole assembly 24 of the well system 10 of FIG. 1 is shown in a perspective view. The example hybrid drill bit 100 includes a bit body 102 with a pin end 104 on one longitudinal end of the bit body 102, a drill end 106 on another longitudinal end of the bit body 102 opposite the pin end 104, and a central bit body axis A-A defining a central longitudinal axis through the center of the bit body 102. The drill bit 100 is rotated about the central bit body axis A-A while drilling. In some instances, the pin end 104 is male and is threaded to mate with a female box at a tubing end of a drill string. The hybrid drill bit 100 includes roller cones 108 rotatably coupled to roller cone arms 110 extending generally along the central bit body axis A-A and beyond an end (e.g., the drill end 106) of the bit body 102. Each of the roller cones 108 rotate on a roller cone axis B-B extending toward (directly or substantially) the central bit body axis A-A. The hybrid drill bit 100 also includes discs 112 rotatably coupled to disc arms 114 extending generally along the central bit body axis A-A and beyond an end (e.g., the drill end 106) of the bit body 102. Each of the discs 112 rotate on a disc axis C-C that is transverse to a radial of the central bit body axis A-A through a center of each of the discs 112. The hybrid drill bit 100 is considered “hybrid” because it has more than one type of cutting structure and/or cutting element, for example, roller cones 108 and discs 112.

Various types of cutting elements and cutting structures may be provided on a hybrid drill bit. In the example shown in FIG. 2, the hybrid drill bit 100 includes two roller cones 108 and two discs 112. The example hybrid drill bit 100 can include additional or different features and components. For example, the drill bit 100 can be provided with a different number of roller cones 108 and discs 112. In some instances, a hybrid drill bit can have one or more discs and one or more roller cones. For example, a hybrid drill bit can have one disc and one roller cone, one disc and a plurality of roller cones, a plurality of discs and one roller cone, or a plurality of discs and a plurality of roller cones.

The roller cone arms 110 and disc arms 114 are shown attached to the bit body 102 with fasteners 116 such that the roller cone arms 110 and disc arms 114 are removable from the bit body 102. In some instances, the roller cone arms 110 and disc arms 114 are an extension of the bit body 102, are welded to or cast with the bit body 102, and/or are connected to the bit body 102 in another way.

In some instances, the roller cones 108 rotate on spindles (not shown) extending from the bit body 102 or roller cone arms 110 along the roller cone axes B-B. Similarly, in some instances, the discs 112 rotate on spindles (not shown) extending from the bit body 102 or disc arms 114 along the disc rotation axes C-C. The roller cones 108 and discs 112 can attach to the spindles via a bearing system to allow rotation of the roller cones 108 about the cone rotation axes B-B and/or discs 112 about the disc axes C-C. The bearing system can include, for example, a seal, ball bearings, a lubrication system, and/or a pressure compensation system.

The roller cones 108 can operate at a number of positions and configurations. In the example shown, the roller cones 108 are radially disposed near an outer lateral periphery of the drill end 106. In some instances, the roller cones 108 are disposed more inward toward the central bit body axis A-A, more outward from the central bit body axis A-A, or in a different position. In some instances, the roller cone axes B-B intersect the central bit body axis A-A, for example, to allow rolling of the roller cones 108 against a rock formation as the example hybrid drill bit 100 rotates about its central bit body axis A-A. Rolling occurs when the roller cone axes B-B intersect the central bit body axis A-A, and a radial vector of the roller cone 108, perpendicular to the roller cone's axis B-B, points in the direction of rotation of the hybrid drill bit 100. In other instances, the roller cone axes B-B do not intersect the central bit body axis A-A, and the roller cone axes B-B are non-radial from the central bit body axis A-A. When a roller cone axis B-B does not intersect the central bit body axis A-A, the roller cone has an additional skidding motion against a formation. In certain instances, a hybrid drill bit can have at least two roller cones, where a first roller cone has a roller cone axis that intersects a central bit body axis, and a second roller cone has a roller cone axis that does not intersect the central bit body axis.

Each of the roller cones 108 includes a cone body 118 and multiple cutting teeth 120 disposed on the cone body 118. In the example shown in FIG. 2, the cutting teeth 120 are wedge-shaped protrusions extending from the cone body 118. In other instances, the cutting teeth 120 are a different shape protrusion on the cone body 118. The cutting teeth 120 can be milled and/or inserts. In certain instances, milled steel teeth, for example, milled from the steel cone body 118, can be provided with a hardface metal coating to withstand crushing forces during operation of the example hybrid drill bit 100. Inserts often include a different material such as carbide, tungsten, diamond, and/or other hard material. In the example shown in FIG. 2, the two roller cones 108 are the same in shape and size. In other instances, one or more of the roller cones on a hybrid drill bit are a different shape or size from each other.

The discs 112 can be provided in a number of positions and configurations. In the example shown, the discs 112 are radially disposed near the outer lateral periphery of the drill end 106. In some instances, the discs 112 are disposed more inward toward the central bit body axis A-A, more outward from the central bit body axis A-A, or in a different position. Each of the discs 112 includes a disc body 122 having multiple disc cutters 126 disposed in a cutting row 124 around the circumference of the disk body 122. The cutting row 124 defines a rotational plane 128 (FIG. 4A) through the center of the cutting row 124, where the rotational axis C-C is normal to the rotational plane 128. The center of the disc is the intersection of the rotational axis C-C and the rotational plane 128. Each disc cutter 126 includes a generally cylindrical cutter body and a flat (substantially or completely) end face at an end of the cylindrical body. Each of the cutters is arranged on a cutter axis D-D. A cutter axis D-D through a center of the cylindrical cutter body is transverse to a radial from the center of the disc 112 to the center of the cylindrical cutter body. In some instances, the cutter axis D-D is parallel to the disc axis C-C. In other instances, the cutter axis D-D is canted at a non-zero angle from the disc axis C-C. One or more (or all) of the cutters 126 can have cutter axes D-D at the same angle to disc axis C-C, or one or more (or all) of the cutters 126 can have cutter axes D-D at different angles. An orientation of the disc cutters 126 on the disc 112 can bias the disc 112 into rotation during operation of the hybrid drill bit 100, where certain orientations are inclined to promote faster rotation of the disc 112, and certain other orientations are inclined to promote slower rotation of the disc 112. In the example shown in FIG. 2, the disc cutters 126 on the discs 112 are oriented similarly to each other about the respective disc axis C-C. For example, during operation of the example hybrid drill bit 100, the discs 112 rotate about their respective disc axes C-C, and the disc cutters 126 orient about the discs 112 such that each disc cutter 126 on a respective disc 112 is oriented similarly as it passes through a fixed, non-rotating point on the disc 112 (i.e., as it contacts the bottom of the bore hole being drilled). In other instances, the disc cutters 126 on one of the discs 112 are oriented differently from each other about their respective disc axis C-C. For example, the disc cutters 126 can be shaped to cycle between varying back rake angles as the disc 112 rotates about its disc axis C-C.

In some instances, the disc cutters 126 are a different shape than the cylindrical shape shown in FIG. 2. For example, the disc cutters 126 can be rectangular, pointed, or another shape. In other instances, a single, continuous cutter is provided around the entire circumference of the body 122.

In the example shown in FIG. 2, the two discs 112 are the same in shape and size. In other instances, one or more of the discs on a hybrid drill bit are a different shape or size from each other. The disc cutters 126 are shown as being cylindrical polycrystalline diamond compact (PDC) cutters partially embedded into the cutting row 124 of the disc 112. The disc cutters 126 can be partially embedded into the disc body 122, coupled to the disc body 122 through welding, with fasteners, bonded, and/or otherwise disposed about the disc body 122. In some instances, the disc cutters 126 are different, for example, natural diamond inserts, thermally stable PDC cutters, tungsten carbide inserts, metal inserts, milled cutters or teeth, or another hard and abrasive material.

In some instances, the bit body 102 includes a nozzle 130 at the drill end 106 to provide drilling fluid to the hybrid drill bit 100 during drilling.

Referring to FIG. 3A, the example hybrid drill bit 100 is shown in an end view, specifically, showing the drill end 106. The roller cone axes B-B extend toward the central bit body axis A-A, and the disc axes C-C are transverse to radials of the central bit body axis A-A through the centers of the discs 112.

In the example shown in FIG. 3A, the roller cone axes B-B intersect the central bit body axis A-A. In some instances, the roller cone axes B-B are non-radial from the central bit body axis A-A, for example, as depicted schematically in FIG. 5. FIG. 5 shows a partial schematic end view of the example hybrid drill bit 100 of FIG. 3A, except the roller cone axis B-B of FIG. 5 is non-radial from central bit body axis A-A. If non-radial, an offset distance, e, between the roller cone axis B-B and the central bit body axis A-A is less than or equal to 9/16 inches in a 10.5 inch size or smaller bit. For example, the offset distance e can be 0.0625 inch, 0.125 inch, or another value. The offset distance e is the shortest distance between the central bit body axis A-A and the roller cone axis B-B. In other words, the offset distance e can be defined as a distance between the roller cone axis B-B and a plane through the central bit body axis A-A, where the plane is parallel to the roller cone axis B-B. In some instances, the offset distance e is the same for each roller cone 108. In other instances, the offset distance e is different for one or more or each roller cone 108. The roller cones 108 act to crush and/or crack a formation in a rotating action with the cutting teeth 120. The cutting teeth 120 crush and/or crack a formation to create fractured cuttings of formation.

In some instances, the offset distance e is small such that while the hybrid drill bit 100 rotates about the central bit body axis A-A and the drill end 106 is against a formation, the roller cones 108 rotate in a rolling motion with a small amount of shear or skidding relative to rolling. In instances with a small offset distance e, the rolling motion with a small amount of shear or skid facilitates drilling into a formation, for example, in drilling into a soft formation. In other instances, the offset distance e is zero (e.g., as depicted in FIG. 3A) such that as the hybrid drill bit 100 rotates about a central bit body axis A-A and the drill end 106 is against a formation, the roller cones 108 rotate in a rolling motion without shear against the formation. In instances with a zero offset distance e, the rolling motion of the roller cones 108 facilitates drilling into a formation, for example, in drilling into a hard formation.

The roller cone axis B-B is at a pin angle, β_(c), relative to a plane perpendicular to the central bit body axis A-A and through the center of the roller cone 108. In the example roller cone 108 shown in FIG. 4B, the pin angle ⊖_(c) is about 30 degrees. The pin angle β_(c) can be different, for example, ⊖_(c) can be 0 degrees, 90 degrees, an angle between 0 and 90 degrees, or another angle.

Referring back to FIG. 5, the disc axis C-C is transverse to a radial of the central bit body axis A-A. A location of each disc 112 on the bit body 102 can be measured by an axis offset, S, and a disc center offset, L. The axis offset, S, is the shortest distance between the disc axis C-C and the central bit body axis A-A. In other words, the axis offset S can be defined as a distance between the disc axis C-C and a plane through the central bit body axis A-A, where the plane is parallel to the disc axis C-C. The disc center offset, L, is the shortest distance between the center of the disc 112 and a plane through the central bit body axis A-A and parallel to axis offset S. The axis offset, S, and the disc center offset, L, orient the disc 112 such that as the hybrid drill bit 100 rotates, the disc axis C-C points in (directly or substantially) the rotational direction of the drill bit 100. Referring back to FIG. 3, the discs 112 each have an axis offset, S, and a disc center offset, L that can be the same or different for each disc 112. In operation, the drill end 106 of the hybrid drill bit 100 presses against a formation, the hybrid drill bit 100 rotates about the central bit body axis A-A, and each disc 112 rotates about its disc axis C-C such that a subset of the disc cutters 126 on each disc 112 are in contact with the formation while the remainder of the disc cutters 126 are not in contact with the formation. As drill bit 100 is rotated on the bit body axis A-A, the discs 112 rotate about the disc axis C-C, and the disc cutters 126 cycle between being in contact with the formation and disengaging the formation. In some instances, cycling the disc cutters 126 in this manner reduces wear on the disc cutters 126 and extends the operational life of the disc 112. As the hybrid drill bit 100 rotates about the central bit body axis A-A and the drill end 106 is against a formation, the discs 112 rotate and the disc cutters 126 shear against the formation. The shearing action of the disc cutters 126 on the disc 112 facilitates drilling into hard rock formations.

The disc axis C-C is at a pin angle, β_(d), relative to a plane perpendicular to the central bit body axis A-A and through the center of the disc 112. In the example disc 112 shown in FIG. 4A, the pin angle β_(d) is about 30 degrees. The pin angle β_(d) can be different, for example, β_(d) can be 0 degrees, 90 degrees, an angle between 0 and 90 degrees, or another angle.

In certain instances, the roller cones 108 and the discs 112 are symmetrically fixed on the bit body 102 such that the hybrid drill bit 100 is balanced during operation. In other instances, the roller cones 108 and the discs 112 are not symmetrically fixed on the bit body 102.

FIG. 3B shows an example hybrid drill bit profile 200 corresponding to the example hybrid drill bit 100 of FIG. 3A. The example hybrid drill bit profile 200 includes the shape cut by the hybrid drill bit 100 of FIG. 3A, showing each associated cutting element of the drill bit 100 transposed on a plane. In the example shown in FIG. 3B, the hybrid drill bit profile 200 is defined by the cutting teeth 120 of the roller cones 108 and an outline 202 of the disc cutters 126 of the discs 112 projected onto a vertical plane passing through the central bit body axis A-A of the bit body 102.

In the example profile 200 shown in FIG. 3A, the cutting teeth 120 on the roller cones 108 extend further in the profile 200 than the outline 202 formed by the disc cutters 126 of the discs 112. The cutting teeth 120 lead the disc cutters 126 such that the cutting teeth 120 prefracture the formation during drilling. The cutting teeth 120 fracture the formation, leaving fractured cuttings of formation and peaks and divots in the formation, and the disc cutters 126 act to scrape the fractured cuttings of formation and substantially level the formation by scraping the peaks and divots left by the cutting teeth 120 such that a trailing roller cone can crush and/or crack a more level formation surface. In some instances, the outline 202 formed by the disc cutters 126 extend further in the profile 200 than the cutting teeth 120 on the roller cones 108. In other instances, a number of cutting teeth 120 on the roller cones 108 extend further in the profile 200 than a portion of the outline 202 formed by the disc cutters 126, and a remaining portion of the outline 202 formed by the disc cutters 126 extends further in the profile 200 than a remainder of the cutting teeth 120 on the roller cones 108. In certain instances, one or more cutting teeth extend further in a profile than one or more disc outlines, and another subset of disc outlines extend further than other cutting teeth. For example, a cutting teeth profile of a first roller cone can extend further than a disc cutter outline, and the disc cutter outline can extend further than a cutting teeth profile of a second roller cone.

The outline 202 formed by the disc cutters 126 of the discs 112 can vary.

In some instances, the outline 202 can form a different curve based on the disc axis offset S, disc center offset L, the pin angle β_(d), and/or another characteristic of the discs 112. In certain instances, roller cone 108 is shaped to substantially match the outline 202 formed by the disc cutters 126. In certain other instances, the outline 202 is shaped to match a shape of the roller cone 108.

Hybrid drill bits, such as the example hybrid drill bit 100, can be configured to rotate about a central bit body axis with a drill end against a formation in a wellbore. Roller cones disposed about the drill end crush or crack a formation in a substantially or wholly rotating action, while disc cutters on a disc scrape against the formation in a shearing action, with the disc configured to rotate. The rotating action of the roller cones includes rolling against the formation, and the shearing action of the discs includes cycling the disc cutters against the formation by rotating the disc. In some instances, the roller cones prefracture the formation such that the disc cutters of the disc scrape the prefractured cuttings of formation.

In instances where the roller cones 108 and discs 112 are symmetrically fixed on the drill end 106 of the bit body 102, symmetry in the roller cones 108 and symmetry in the discs 112 promote a force balance and/or energy balance on the hybrid drill bit 100.

In view of the discussion above, certain aspects encompass, a hybrid drill bit including a bit body defining a central bit body axis, a roller cone at an end of the bit body and rotatably coupled to the bit body about a roller cone axis, and a disc at the end of the bit body and rotatably coupled to the bit body about a disc axis. The roller cone axis extends towards the central bit body axis. The disc axis is transverse to a radial of the central bit body axis through a center of the disc, and the disc includes a plurality of disc cutters.

Certain aspects encompass, a well drilling system including a well head, a drill string connected to the well head, and a hybrid drill bit connected to the drill string. The hybrid drill bit includes a bit body defining a central bit body axis, a roller cone at an end of the bit body and rotatably coupled to the bit body about a roller cone axis, and a disc at the end of the bit body and rotatably coupled to the bit body about a disc axis. The roller cone axis extends toward the central bit body axis, and the disc axis extends away from the central bit body axis.

Certain aspects encompass, a method of cutting a formation in a wellbore. The method includes rotating a drill bit in a formation in a wellbore, crushing or cracking the formation in a rotating action using a roller cone on the drill bit, and scraping the formation in a shearing action using disc cutters on a disc on the drill bit, with the disc configured to rotate.

The aspects above can include some, none, or all of the following features. The hybrid drill bit includes a first arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body and a second arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body. The disc is rotatably coupled to the first arm, and the roller cone is rotatably coupled to the second arm. The roller cone axis intersects the central bit body axis. The roller cone axis is non-radial from the central bit body axis. The shortest distance between the central bit body axis and the roller cone axis is less than 9/16 inches. The roller cone includes a cone body and a plurality of cutting teeth disposed on the cone body. The cutting teeth of the roller cone include milled steel teeth with a hardface metal coating. The plurality of disc cutters on the disc extend further from the end of the bit body than the plurality of cutting teeth on the roller cone. The plurality of cutting teeth on the roller cone extends further from the end of the bit body than the plurality of disc cutters on the disc. The location of the disc is determined by offset distance L and offset distance S. The disc includes a disc body and a generally ring-shaped cutting row radially disposed about the disc body, and the plurality of disc cutters is disposed along the generally ring-shaped cutting row. Each disc cutter in the plurality of disc cutters includes a cylindrical cutter body with a cutter axis through a center of the cylindrical cutter body and a flat cutter face at an end of the cylindrical body. The cutter axis is transverse to a radial from the center of the disc to the center of the cylindrical cutter body. The hybrid drill bit includes a plurality of roller cones and a plurality of discs. Each disc of the plurality of discs has a different disc axis offset, S, and a different disc center offset, L, from the central bit body axis. Each disc axis of the plurality of discs is at a different angle β from the central bit body axis, where the angle β is the angle of the respective rotational axis from the central bit body axis. Crushing or cracking the formation in a rotating action using a roller cone includes prefracturing the formation using the roller cone, and the roller cone is deeper into the formation than the disc cutters of the disc. Scraping the formation in a shearing action using disc cutters on a disc includes scraping prefractured cuttings of formation. Scraping the formation in a shearing action using disc cutters on a disc on the drill but includes rotating the disc about a disc axis such that the disc cutters cycle about the disc axis.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be. Accordingly, other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A hybrid drill bit, comprising: a bit body defining a central bit body axis; a roller cone at an end of the bit body and rotatably coupled to the bit body about a roller cone axis, the roller cone axis extending toward the central bit body axis; and a disc at the end of the bit body and rotatably coupled to the bit body about a disc axis; where the disc axis is transverse to a radial of the central bit body axis through a center of the disc; and where the disc comprises a plurality of disc cutters.
 2. The hybrid drill bit of claim 1, further comprising: a first arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body, where the disc is rotatably coupled to the first arm; and a second arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body, where the roller cone is rotatably coupled to the second arm.
 3. The hybrid drill bit of claim 1, where the roller cone axis intersects the central bit body axis.
 4. The hybrid drill bit of claim 1, where the roller cone axis is non-radial from the central bit body axis.
 5. The hybrid drill bit of claim 4, where the shortest distance between the central bit body axis and the roller cone axis is less than 9/16 inches.
 6. The hybrid drill bit of claim 1, where the roller cone comprises a cone body and a plurality of cutting teeth disposed on the cone body.
 7. The hybrid drill bit of claim 6, where the cutting teeth of the roller cone comprise milled steel teeth with a hardface metal coating.
 8. The hybrid drill bit of claim 6, where the plurality of disc cutters on the disc extend further from the end of the bit body than the plurality of cutting teeth on the roller cone.
 9. The hybrid drill bit of claim 6, where the plurality of cutting teeth on the roller cone extend further from the end of the bit body than the plurality of disc cutters on the disc.
 10. The hybrid drill bit of claim 1, where the disc comprises a disc body and a generally ring-shaped cutting row radially disposed about the disc body; and where the plurality of disc cutters are disposed along the generally ring-shaped cutting row.
 11. The hybrid drill bit of claim 10, where each disc cutter in the plurality of disc cutters comprises a cylindrical cutter body with a cutter axis through a center of the cylindrical cutter body and a flat cutter face at an end of the cylindrical body; and where the cutter axis is transverse to a radial from the center of the disc to the center of the cylindrical cutter body.
 12. The hybrid drill bit of claim 1, comprising a plurality of roller cones and a plurality of discs.
 13. The hybrid drill bit of claim 12, where each disc of the plurality of discs has a different disc axis offset, S, and a different disc center offset, L, from the central bit body axis; and where each disc axis of the plurality of discs is at a different angle β from the central bit body axis, where the angle β is the angle of the respective rotational axis from the central bit body axis.
 14. A well drilling system, comprising: a well head; a drill string connected to the well head; a hybrid drill bit connected to the drill string, the hybrid drill bit comprising: a bit body defining a central bit body axis; a roller cone at an end of the bit body and rotatably coupled to the bit body about a roller cone axis, the roller cone axis extending toward the central bit body axis; and a disc at the end of the bit body and rotatably coupled to the bit body about a disc axis, the disc axis extending away from the central bit body axis.
 15. The well drilling system of claim 14, where the hybrid drill bit further comprises: a first arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body, where the disc is rotatably coupled to the first arm; and a second arm coupled to the bit body extending generally along the central bit body axis and beyond the end of the bit body, where the roller cone is rotatably coupled to the second arm.
 16. The well drilling system of claim 14, where the roller cone comprises a cone body and a plurality of cutting teeth disposed on the cone body; where the disc comprises a disc body and a generally ring-shaped cutting row radially disposed about the disc body; and where the generally ring-shaped cutting row includes a plurality of disc cutters disposed along the cutting row.
 17. A method of cutting a formation in a wellbore, the method comprising: rotating a drill bit in a formation in a wellbore; crushing or cracking the formation in a rotating action using a roller cone on the drill bit; and scraping the formation in a shearing action using disc cutters on a disc on the drill bit, the disc configured to rotate.
 18. The method of claim 17, where crushing or cracking the formation in a rotating action using a roller cone comprises prefracturing the formation using the roller cone, where the roller cone is deeper into the formation than the disc cutters of the disc; and where scraping the formation in a shearing action using disc cutters on a disc comprises scraping prefractured cuttings of formation.
 19. The method of claim 17, where scraping the formation in a shearing action using disc cutters on a disc on the drill bit further comprises rotating the disc about a disc axis such that the disc cutters cycle about the disc axis. 